Here are a few observations on the conflict between w3.org standards and some XML feed validators, RSS Feed Readers or News Aggregators as most people know them, which mushroomed in the recent years. Some of the elements of your document will still not be able to be read properly.

Below is an explanation about what Relative URL is and why it is used in your HTML document according to [RFC1808]:

This document describes the syntax and semantics for "relative"Uniform Resource Locators (relative URLs): a compact representation of the location of a resource relative to an absolute base URL. It is a companion to RFC 1738, "Uniform Resource Locators (URL)" [2], which specifies the syntax and semantics of absolute URLs.

A common use for Uniform Resource Locators is to embed them within a document (referred to as the "base" document) for the purpose of identifying other Internet-accessible resources. For example, in hypertext documents, URLs can be used as the identifiers for hypertext link destinations.

Absolute URLs contain a great deal of information which may already be known from the context of the base document's retrieval, including the scheme, network location, and parts of the URL path. In situations where the base URL is well-defined and known, it is useful to be able to embed a URL reference which inherits that contex rather than re-specifying it within each instance. Relative URLs can also be used within data-entry dialogs to decrease the number of characters necessary to describe a location.

Contrary to the established practice of World Wide Web Consortium on the use of Realtive URLs, some XML readers are unable to publish the images embeded via this method.

Some RSS elements are allowed to contain HTML, including links and images. However, if these links use relative URLs to point to resources, users of some news aggregators will not be able to click on them. And your images use the relative URLs, the images may not appear (since the news aggregator will be looking for the images locally on the user's hard drive, rather than on your web site).

How one should act in this situation? If you know that the content that you publish will be viewed via RSS Readers, either hot link the images to their original sites, or use a separate server to upload your images to and provide a full URL to your images versus a Realative. For example [url=http://yourdomain/images/picture.jpg]http://yourdomain/images/picture.jpg[/url] versus /images/picture.jpg.